The need in the field of magnetic data storage for increased data storage capacity in data storage devices, such as magnetic hard disk drives (HDDs), requires that the areal-density of these devices, defined by the data can be stored in a given area of magnetic disk, be increased. Due to its advantages in increasing areal-density over longitudinal recording systems, which use the in-plane orientation of magnetic moments, perpendicular magnetic recording systems, which employ the alignment of magnetic moments perpendicular to the disk surface, are already in production. For a HDD, areal-density is the product of bit-density (number of bits per unit length along the track-direction) and track-density (number of tracks per unit length along the radial direction). Areal-density can be increased either by increasing bit-density and/or track-density. In general, both the bit-density and track-density are increased to achieve higher areal-density.
Among the technologies that have so far contributed to increasing the areal-density of HDDs, the application of magneto-resistive sensors as read heads holds tremendous promise. A reproducing element used in many current HDDs employs a magneto-resistive effect, a phenomenon where electric resistance of the element changes due to a change in a magnetic orientation of the magnetic field emanating from a magnetic disk medium. In general, a giant magneto-resistance (GMR) or a tunnel magneto-resistance (TMR) element may be used in current HDDs.
Referring to FIG. 1, a GMR or a TMR element possesses a spin-valve structure, comprised of a “pinned layer” 42 with magnetic moment 42a aligned to a particular direction, and a “free layer” 43 with magnetic moment 43a adapted to rotate freely along the film plane. Rotation of the magnetic moments in the free layer due to the magnetic field 9a emanating from the magnetic disk medium 9 makes an angle θ44 between the magnetic moments of the free layer and the pinned layer. This angular difference, which is detected through a change of electric resistance of the element, changes by the upward and downward magnetic flux from the medium enabling the detection of the magnetization orientation in the written bit on the magnetic medium. Change of electric resistance is measured in the form read-voltage, in most cases.
In order to increase the track-density of a magnetic medium, which is one method to achieve higher areal-density, the track-width 41 and track pitch defined by the distance between the two adjacent tracks may be decreased. An intensity of magnetic flux that emanates from a track in the medium becomes greatest at the track center, where the bits are well-written and not influenced by surrounding effects. As the distance from the track center increases, the magnetization pattern emanated from the track gets distorted this distortion becomes greatest (the pattern is the most distorted) at the track-edge region, where all of the magnetizations in the medium do not necessarily align perfectly along the upward or downward direction (e.g., there are magnetizations in a direction not perpendicular to the plane of the medium surface).
At the track center, read sensitivity of a read head becomes greatest while it runs along a cross-track direction. Read-voltage signals become smaller at the track-edge region due to the distortion of magnetization alignment. Moreover, total magnetic flux emanating from the medium for a track decreases with the decrease of track width, resulting in a decrease of the read-voltage, an effect experienced even at the track-center. While a narrow track is read by a sensor element with fixed width, it senses “unexpected” signals from the track-edge region resulting in a decrease of read-voltage even when the sensor is positioned at the track-center. Moreover, a reduction of track pitch brings two adjacent tracks closer to each other, which causes the problem of side reading even when a read sensor is positioned on an adjacent track.
A reduction of sensor-width 40 helps to overcome the problem of sensing the signals from the track-edge and adjacent tracks. However, a simple reduction of sensor-width results in another problem. In general, an initial magnetization state of the free layer of a sensor element is defined by applying a magnetic field from an adjacent layer, referred to as “hard bias.” Influence of a hard bias field, which is applied along the cross-track width direction, is stronger at the two edges of the free layer 43 than at the region away from the edge (i.e., around the middle portion of the sensor). For a fixed value of a hard bias field, sensitivity (how freely the magnetization can rotate) of the free layer is reduced while sensor-width is narrowered, which is due to the increase of the region strongly influenced by the hard bias field. A simultaneous reduction of the hard bias magnetic field is helpful to keep the sensitivity of the sensor to a certain degree. However, excessive reduction of the hard bias field makes the free layer more sensitive to the magnetization in the neighboring track, which increases side reading.
A method to improve read element sensitivity while sensor-width is reduced is described in U.S. Pat. No. 7,106,560 and U.S. Patent Application Publication No. US2004/0012899. As described in these references, the bias magnetic field applied to the free layer varies along the sensor height—e.g., the bias magnetic field applied to a region away from the air bearing surface is smaller than that applied in a region close to the air bearing surface. However, for further decrease of sensor width, read head sensitivity in the region away from the air bearing surface might be reduced unless a bias magnetic field is changed, and in such a case, this attempt may not be able to keep the read head sensitivity to a satisfactory level. Further reduction of the bias magnetic field might help to keep the read head sensitivity, but this ultimately may not be sufficient for use with the extremely narrow sensor width.
In a conventional read sensor element, which possesses magnetic films with in-plane magnetic anisotropy, magnetizations are aligned along the cross-track direction (as illustrated in FIG. 1) partly with the help of shape anisotropy. While sensor-width is reduced, in-plane magnetization orientation becomes unstable and a circular magnetization mode is preferable to minimize the total magnetic energy of the element. Under such circumstances, sensitivity of the read head deteriorates drastically and becomes unable to cope with the extremely narrow track width.
In U.S. Pat. No. 6,910,382, a magnetic-semiconductor based sensor device is proposed that has a high-sensitivity sensor to detect magnetization switching behavior in magnetic recording devices. The proposed device employs the detection of voltage change due to a galvanomagnetic phenomenon referred to as a “planar Hall effect.” Unlike metallic ferromagnetic materials, magnetic-semiconductor materials proposed in this reference exert performance in an extremely low (such as about 20K) temperature region. In addition, in realistic magnetic recording devices (such as HDDs), operation under normal room temperature (such as in a range of 293K-303K) is a necessary condition. Moreover, the change of voltage due to planar Hall effect described in this patent is basically non-linear in nature with respect to an external magnetic field. The method requires the total switching of the magnetization of the sensor part to detect the magnetization pattern written on a magnetic disk medium. On the other hand, in current HDDs, a linear part of the head sensitivity criteria (voltage or resistance change with respect to an external magnetic field) is used, and thus, instead of taking advantage of the total magnetization switching, rotation of magnetization (a phenomenon before the magnetization switches totally to the direction opposite to its initial orientation) of the sensor may be employed.
Therefore, a read sensor with high sensitivity that is capable of reproducing the data written on a narrow track of a magnetic medium would be beneficial to increase both track-density and areal-density of the magnetic medium.